The invention relates to a low-pressure mercury-vapor discharge lamp comprising a discharge vessel having a first and a second end portion,
wherein the discharge vessel encloses a discharge space containing a filling of mercury and an inert gas in a gastight manner,
wherein the end portions each support an electrode arranged in the discharge space which electrode is used to generate and maintain a discharge in the discharge space,
wherein current supply conductors of the electrodes extend through the end portions so as to project from the discharge vessel,
and wherein an electrode shield at least substantially surrounds at least one of the electrodes.
In mercury-vapor discharge lamps, mercury is the primary component for (efficiently) generating ultraviolet (UV) light. An inner surface of the discharge vessel may be provided with a luminescent layer containing a luminescent material (for example a fluorescent powder) for converting UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sunbed lamps) or to visible radiation for general lighting purposes. Such discharge lamps are therefore also referred to as fluorescent lamps. The discharge vessel of low-pressure mercury-vapor discharge lamps is generally cylindrical in shape with a circular cross-section and comprises both elongated and compact embodiments. In general, the tubular discharge vessel of so-called compact fluorescent lamps comprises a collection of relatively short, straight parts having a relatively small diameter, which straight parts are interconnected by means of bridge pieces and/or via curved pieces. Compact fluorescent lamps are generally provided with an (integrated) lamp cap.
A low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph is known from DE-A 1 060 991. In said known lamp, the electrode shield surrounding the electrode is made from thin sheet titanium and is supported by a supporting wire which is anchored to the end portion. By using an electrode shield, which is also referred to as anode shield or cathode shield, blackening at an inner surface of the discharge vessel is counteracted. In this respect, titanium serves as the getter for chemically binding oxygen, nitrogen and/or carbon. The supporting wire serves to keep the electrode shield in place.
A drawback resides in that mounting of the electrode shield is fairly complicated.
It is an object of the invention to provide a low-pressure mercury-vapor discharge lamp of the type mentioned in the opening paragraph, which can be manufactured more readily and more economically. A further object of the invention is to provide a low-pressure mercury-vapor discharge lamp having a relatively low mercury consumption.
To achieve this, the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is mounted on the current supply conductors.
Since the electrode shield is supported by the current supply conductors, a supporting wire for keeping the electrode shield in place is not necessary. Often, use is not only made of a supporting wire which is anchored to the end portion of the discharge lamp, but also of a support which comprises (a part of) the electrode shield, which support is connected to the supporting wire. The construction in accordance with the invention enables a support and a supporting wire to be omitted, and, during the manufacture of the discharge lamp, it is no longer necessary to provide the supporting wire in the end portion. As a result, the low-pressure mercury-vapor discharge lamp in accordance with the invention can be manufactured more readily and more economically.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is clamped to the current supply conductors. The advantage of this construction resides in that the electrode shield is provided in a predetermined place on the current supply conductors, so that the electrode shield surrounds the electrodes in the desired manner. Clamping the electrode shield to the current supply conductors helps to hold the electrode shield in place during the service life of the discharge lamp, irrespective of the position of said discharge lamp. If the electrode shield is, for example, tubular, it is desirable for the electrode shield to be positioned at least substantially symmetrically with respect to the electrode. During the manufacture of the discharge lamp, the electrode shield is slid over the current supply conductors until it is in a predetermined position.
An alternative embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the current supply conductors are flattened, at the location of the mounted electrode shield, in a plane parallel to the electrodes. By flattening a part of the current supply conductors at the location of the mounted electrode shield, it becomes possible to mount the electrode shield in a predetermined position without exerting a clamping force. Said flat part in the current supply conductors helps to hold the electrode shield in place during the service life of the discharge lamp, irrespective of the position of the discharge lamp.
In a further alternative, favorable embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention, the electrode shield is provided with an incision at the location of the current supply conductors. During the manufacture of the discharge lamp, the current supply conductors are bent outwards, for example to provide the electrodes with an electron-emitting substance. Before the current supply conductors are bent back to the desired position, the electrode shield is provided, and the current supply conductors are positioned in the incisions in the electrode shield. The width of the incisions in the electrode shield may be such that the electrode shield is mounted so as to be a press fit on the current supply conductors.
Preferably, the electrode shield is provided with a slit on a side facing the discharge space. A slit in the electrode shield in the direction of the discharge brings about a relatively short discharge path between the electrodes of the low-pressure mercury-vapor discharge lamp. This is favorable for obtaining a high-efficiency lamp. The slit extends preferably in a direction parallel to the axis of symmetry of the electrode shield (so-called lateral slit in the electrode shield). In the known lamp, the opening or slit in the electrode shield faces away from the discharge space.
A preferred embodiment of the low-pressure mercury-vapor discharge lamp in accordance with the invention is characterized in that the electrode shield is made from a ceramic material.
To obtain properly functioning low-pressure mercury-vapor discharge lamps, the electrodes of such discharge lamps comprise an (emitter) material with a low so-called work function (reduction of the work function voltage) to supply electrons to the discharge (cathode function) and receive electrons from the discharge (anode function). Known materials having a low work function are, for example, barium (Ba), strontium (Sr) and calcium (Ca). It has been observed that, during operation of the low-pressure mercury-vapor discharge lamp, material (barium and strontium) evaporates from the electrode(s). In general, the emitter material is deposited on the inner wall of the discharge vessel. It has further been found that the above-mentioned Ba (and Sr), which is deposited elsewhere in the discharge vessel, no longer participates in the light-generation process. The deposited (emitter) material further forms mercury-containing amalgams on the inner wall, causing the quantity of mercury available for the discharge to decrease (gradually), which may adversely affect the service life of the lamp. In order to compensate for such mercury loss during the service life of the lamp, a relatively high quantity of mercury is necessary in the lamp, which is undesirable from the point of view of environmental protection. The provision of an electrode shield, which surrounds the electrode(s) and is made from a ceramic material, reduces the reactivity of materials in the electrode shield relative to the mercury in the discharge vessel, leading to the formation of amalgams (Hgxe2x80x94Ba, Hgxe2x80x94Sr). In addition, the use of an electrically insulating material precludes the development of short circuits of the pole wires of the electrode(s) and/or of a number of windings of the electrode(s). The known lamp has an electrode shield of an electroconductive material, which, in addition, relatively readily forms an amalgam with mercury. The mercury consumption of the discharge lamp is limited by substantially reducing the degree to which the material of the shield surrounding the electrode(s) reacts with mercury.
The electrode shield itself should not appreciably absorb mercury. To achieve this, the material of the electrode shield includes at least an oxide of at least one element of the series formed by magnesium, aluminium, titanium, zirconium, yttrium and the rare earths. Preferably, the electrode shield is made from a ceramic material which comprises aluminium oxide. Particularly suitable electrode shields are manufactured from so-called densely sintered Al2O3, also referred to as DGA. An additional advantage of the use of aluminium oxide is that an electrode shield made of such a material is resistant to relatively high temperatures. At such relatively high temperatures, there is an increased risk that the (mechanical) strength of the electrode shield decreases, thus adversely affecting the shape of the electrode shield. If a metal or a metal alloy is used as the electrode shield, as is the case in the known discharge lamp, the temperature of the electrode shield must not be too high to prevent that the metal or one of the metals of the metal alloy begins to deform or evaporate, thereby giving rise to undesirable blackening of the inner surface of the discharge vessel. (Emitter) material originating from the electrode(s) and deposited on an electrode shield of aluminium oxide which is at a much higher temperature, cannot, or hardly, react with the mercury present in the discharge, as a result of said high temperature, so that the formation of mercury-containing amalgams is at least substantially precluded. In this manner, the use of an electrode shield in accordance with the invention serves a dual purpose. On the one hand, it is effectively precluded that material originating from the electrode(s) is deposited on the inner surface of the discharge lamp, and, on the other hand, it is precluded that (emitter) material deposited on the electrode shield forms amalgams with the mercury present in the discharge lamp. In addition, Ba, Sr and Ca may react with Al2O3 forming the corresponding aluminates which no longer bind Hg. Preferably, in operation, the temperature of the electrode shield exceeds 250xc2x0 C. An advantage of such a relatively high temperature is that, in particular, in the initial stage, the electrode shield becomes hotter than in the known lamp, as a result of which any mercury bound to the electrode shield is released more rapidly and more readily.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.